This invention relates to extruders of the type in which a screw, rotatable within a barrel, is employed to extrude material through a die or into an injection mold connected to the outlet end of the barrel. The invention is concerned particularly with improvements in high output plasticating extruders.
A plasticating extruder receives polymer pellets or powder (often together with formulation additives in liquid or particle form), works and raises the temperature of the polymer sufficiently to dispose it in a melted or plastic state, and delivers the melted polymer under pressure through a restricted outlet or die. Ordinarily, it is desirable that the discharge extrudate be fully melted, well mixed, uniform in temperature and pressure, and substantially free of small jells and other fine structure agglomerations. It is also desirable that the rate of delivery of the molten polymer through the die be regulated simply by changing the rate of extruder screw rotation and that the rate of delivery of the selected screw speed be substantially uniform.
The basic extruder apparatus includes an elongated barrel which may be heated or cooled at various locations along its length and a screw which extends longitudinally through the barrel. The screw has a helical conveying flight on its surface which cooperates with the cylindrical internal surface of the barrel to define an elongated helical channel.
A typical modern extruder screw includes a plurality of sections configured specifically to the attainment of particular functions. One common design includes a feed section, transition section, and a multi-channel wave metering section. The feed section extends beneath and forwardly from a feed opening where polymer in pellet or powder form is introduced into the extruder to be carried forward along the inside of the barrel by the feed section of the screw. As the polymer material is advanced along the channel, it is worked by the helical threads and barrel. This, in turn, generates heat, and melting of the polymer proceeds as the material is moved along the feed section.
Downstream from the feed section is a conventional single channel transition section of decreasing channel depth in which melting of the material continues as it moves therethrough. Actually, the melting takes place for the most part near the barrel surface at an interface between a film of molten polymer and a solid bed of packed particle polymer. The thin layer of melt film sticks to the barrel wall, but is scraped off by the rotating screw flight and therefore collects in front of the flight. Thus, a somewhat stratified channel composition develops in which a solids bed is positioned at the trailing side of the flight whereas the melt is positioned at the front side, or pushing side of the flight. Toward the end of the transition section, however, after about 40% to 70% of the polymer has been melted, some of the solid bed breaks up and small particles of solid polymer become dispersed in the body of molten polymer.
A partially stratified composition exists at the end of the transition section and is emptied into the multichannel wave metering section, whose function is to exert a pumping action on the molten polymer. The multichannel wave metering section is a melter and a mixer. Typically, two helical flow channels extend through the metering section and are separated by a barrier flight. Each flow channel includes repeating wave cycles of varying depth, having alternating wave crests and valleys. The wave cycles of the adjacent channels are typically helically offset so that a wave crest of one channel lies opposite a valley in the adjacent channel and vice versa. The barrier flight is undercut in a manner which facilitates the flow of melt thereacross from one channel to the other, while restricting the flow thereacross of unwanted solids. Solids therefore tend to pass instead through a restriction formed by the wave crest while the melt travels across the barrier flight into the adjacent channel, thereby minimizing the formation of pressure pulses in the extrudate flow. Such a double wave design is well known and is disclosed in U.S. Pat. No. 4,173,417 which is assigned to the assignee of the present invention, and is incorporated herein by reference.
Improvements in the above-described design as disclosed in U.S. Pat. No. 4,173,417 have been directed to the multi-channel wave metering section, while leaving the conventional transition section unchanged. For example, U.S. Pat. No. 4,925,313 which is assigned to the assignee of the present invention, discloses a double wave metering section having two barrier flights which include segments that are of normal height and are in relatively close clearance with the inner wall of the barrel. The two barrier flights also include undercut segments which form larger gaps between the barrel wall than do the normal segments. Thus the normal height barrier flight segments are separated by undercut segments which extend from one peak to the next peak of the adjacent channel. U.S. Pat. No. 5,035,509, assigned to the assignee of the present invention, discloses a double wave metering section wherein the barrier flight forms a zig-zag shape which divides the helical passage into channels of varying cyclic depth in the helical direction of the channels. Further developments include a triple channel wave screw as disclosed in U.S. Pat. No. 5,219,590, also assigned to the assignee of the present invention. As noted, all of these improvements are directed to the multi-channel wave metering section of the screw while leaving the design of the conventional transition section unchanged.
A second common extruder screw design, referred to as a "barrier screw," utilizes the well established mechanism by which most of the melting occurs near the barrel surface. Such a screw includes a feed section, a barrier melting section and a metering section, the barrier melting section and metering section performing somewhat different functions than the transition section and the multichannel wave metering section described above.
The barrier melting section begins at the terminal end of the feed section, whereby a barrier flight is introduced intermediate the helical thread of the primary flight, typically branching from the primary flight at an increased pitch. The increased pitch of the barrier flight typically continues for one or more turns until the barrier flight is located in a pre-determined position in the channel formed by the primary thread and the barrier thread. Thereupon, either the main flight or the barrier flight changes pitch so that the two flights are parallel throughout the remainder of the barrier melting section.
Thus, the barrier melting section is comprised of two adjacent helical channels, a solids channel and a melt channel, with the barrier flight disposed therebetween. As the screw rotates, the thin melt film which develops at the outer periphery of the solids channel is conveyed over the barrier flight and into the melt channel. The barrier flight is "undercut," providing increased clearance with the barrel wall to facilitate the conveyance of melt thereover. In this manner, melted material is continuously conveyed from the thin melt film into the melt channel, thereby encouraging further solids to melt into the melt film.
Traversing downstream, then, the melt channel begins to fill, whereas the solids channel empties. Barrier melting sections are designed accordingly so that as one traverses downstream, the melt channel deepens whereas the solids channel becomes more shallow, thereby accommodating the increased amount of melt material and the decreased amount of solid material in the respective channels. Barrier screws are well known and examples are shown in U.S. Pat. No. 3,698,541 to Barr, and U.S. Pat. No. 3,858,856 to Hsu.
A "barrier screw" is typically designed to melt the majority of the output in the barrier section, and is made as long as is necessary to accomplish the majority of the melting. Significantly, the clearance of the barrier flight is adapted so that solid material cannot cross thereover and enter the melt channel. Accordingly, traversing downstream along the barrier section, two distinct channels emerge, one of which conveys predominantly melted material and the other of which conveys predominantly solids material. Strict design of barrier clearances and entry and exit from the barrier section are necessary in order to keep the melt material separate from the solid material.
A conventional barrier melting section normally exits into a single channel metering section of constant shallow depth. Because melting is accomplished primarily in the barrier section, the metering section in a barrier melting screw is designed merely to provide uniform output approximately proportioned to the screw rotational speed, i.e., to deliver high quality polymer melt at a uniform rate. Accordingly, each turn of the constant depth metering section reinforces the flow characteristics generated by adjacent turns, and serves to facilitate this goal.
Improvements in the above described "barrier screws" have been directed to enhancing the melting efficiency of the barrier section.
Screws of the two designs described above have improved considerably over the past quarter century. However, it is desirable to achieve a more efficient extruder screw, one in which output quantity and quality is further improved.